Total differential methods based universal post processing algorithm considering geometric error for multi-axis NC machine tool

Multi-axis numerical control machining for free-form surfaces needs CAD/CAM system for the cutter location and orientation data. Since these data are defined with respect to the coordinate of workpiece, they need converting for machine control commands in machine coordinate system, through a processing procedure called post processing. In this work, a new universal post processing algorithm considering geometric error for multi-axis machine tool with arbitrary configuration. Firstly, ideal kinematic model and real kinematic model of the multi-axis NC machine tool are built respectively. Difference between the two kinematic models is only whether to consider the machine tool's geometric error or not. Secondly, a universal generalized post processing algorithm containing forward and inverse kinematics solution is designed to solve kinematic models of multi-axis machine tool. Specially, the inverse kinematics solution is used for the ideal kinematic model, while the forward kinematics solution is used for the real kinematic model. Then, a total differential algorithm is applied to improve the calculation speed and reduce the difficulty of inverse kinematics solution. Realization principle of the total differential algorithm is to transform the inverse kinematics solution problem into that one of solving linear equations based on spatial relationship of adjacent cutter locations. Thirdly, to reduce the complexity of geometric error calibration experiment, effect weight of geometric error components is determined by the sensitivity analysis based on orthogonal method, and then the real kinematic model considering geometric error is established. Finally, the universal post processing algorithm based on total differential methods is implemented and demonstrated experimentally in a five-axis machine tool. The results show that the maximum error value can be decreased to one-fifth using the proposed method in this paper.     

Development of a mobile fused deposition modeling system with enhanced manufacturing flexibility

Development of a flexible and mobile fused deposition modeling (FDM) system from an existing FDM system to enable deposition of material on virtually any surface without being confined to a build chamber is described. Flexibility of the system was demonstrated by depositing ABS on different surfaces, and simple pull tests were performed to determine bonding strength between the deposited materials. To develop the flexible FDM system, a Stratasys FDM 3000 machine was used and modified by reversing the z stage and attaching the x-y table controlling the FDM head to the bottom of the z stage. In this new configuration, the z stage transports the x-y table vertically, and the x-y table controls the x-y motion of the FDM dispensing head, which is exposed at the bottom of the machine. The mean of the absolute value of the difference between 49 part dimensions (based on 20 part features) measured on a modified Grimm test part (n = 5) was ∼0.44 mm for parts fabricated using the developed flexible FDM system, while a mean of ∼0.11 mm was measured using parts produced by the commercial system (n = 5). The dimensional accuracy of the flexible system was comparable but expectedly larger than the commercial system, due to the configuration of the flexible FDM system with the x-y table attached at the bottom of the z stage. There are many possible design improvements particularly focused on reducing deflections in the mechanical components that can be explored and implemented to improve the overall dimensional accuracy of the flexible system, but these investigations are left for future research. Instead, manufacturing flexibility of this new configuration was demonstrated by successfully building a cylinder on flat and 3D cupped surfaces, including building a horizontally oriented cylinder on a wall by orienting the FDM system in the horizontal position. Pull tests were performed and showed that bonding strength for the cylinders built on flat surfaces compared favorably to a glued part (3.06 ± 1.38 MPa for the specimens manufactured with the flexible FDM system compared with 2.00 ± 1.06 MPa for the glued specimens). Additional flexibility was demonstrated by printing directly on a complex curved surface, thus illustrating the possibilities for using AM (a traditional 2D layer-stacking processing technique) in conformal printing applications. It is concluded that this new machine can provide enormous flexibility in freeform manufacturing with applications in part repair, 3D conformal adhesive dispensing, and a number of applications where the removal of the size constraints imposed by the build chamber enables one to deposit new arbitrary features directly on existing parts.     

Precision, Stability and Productivity Increase in Throughfeed Centerless Grinding

Centerless grinding is a high precision manufacturing process commonly applied to the mass production of many industrial components. However, workpiece roundness is critically affected by geometric lobing and no practical tool has been developed to solve the problem in throughfeed working mode. Based on simulation methods previously applied to plunge grinding, a new software tool has been developed in this work. The software determines the optimal working configuration and can be used to reduce set-up time and improve three important features: 1) Precision, as the roundness error is rapidly corrected at the optimal configuration. 2) Productivity, since the workpiece stock can be significantly reduced. 3) Stability, because the process is less sensitive to the original roundness error of the workpiece.     

基于多体系统理论的矫直机几何误差的建模与补偿

针对矫直机的几何误差建模和补偿技术进行研究。将矫直机床视为一个多体系统结构,完成对矫直机的拓扑结构描述,并进行压头、夹头和跨距调整等关键运动部件的误差分析。通过其次变换矩阵体系统推导机床几何误差模型,基于建立的误差模型,采用激光干涉仪识别机床运动轴的几何位置误差。提出矫直机的几何误差补偿方法并设计与开发补偿软件,对矫直机定位精度进行补偿。通过实验进行了误差补偿效果评价,实验结果证明误差模型的正确性以及软件补偿的可行性。     

A strategy for identifying static deviations in universal spindle head type multi-axis machining centers

This research was conducted in order to investigate the effectiveness of the checking method specified in ISO 10791-6 and to propose an additional method for identifying the geometric deviations inherent to five-axis machining centers with a universal spindle head. The specified method is not sufficient for assessing the influence of individual geometric deviations. Therefore, additional measurements are needed in order to use this checking method effectively. The spherical motion in the additional measurements is a modified version of the ISO standard. Some of the deviations are presumed based on the result of the additional measurements, and so the method of identifying the remainder of the geometric deviations was designed by analyzing the link geometry of the machine. The identification procedure consists of three steps. In the first and second steps, after performing two measurements, six out of ten deviations are identified by means of an observation equation. In the third step, four of the ten deviations are identified by means of the link geometry. The exactness of the proposed procedure for identifying the deviations inherent to the five-axis machine is confirmed through simulations.     

Application of Taguchi and response surface methodologies for geometric error in surface grinding process

The geometric error in the surface grinding process is mainly affected by the thermal effect and the stiffness of the grinding system. For minimizing the geometric error, the selection of grinding parameters is very important. This paper presented an application of Taguchi and response surface methodologies for the geometric error. The effect of grinding parameters on the geometric error was evaluated and optimum grinding conditions for minimizing the geometric error were determined. A second-order response model for the geometric error was developed and the utilization of the response surface model was evaluated with constraints of the surface roughness and the material removal rate. Confirmation experiments were conducted at an optimal condition and selected two conditions for observing accuracy of the developed response surface model.     

一种基于误差控制的曲线逼近数控加工策略研究

针对产品的非圆曲线几何轮廓加工,提出一种基于逼近误差的最佳逼近点搜索算法,实现了非圆曲线特征信息完整保留。研究了集成曲线逼近加工策略的数控自动编程系统,能自动识别非圆曲线、圆弧以及直线,自动生成的数控加工代码可在多系统的数控机床上应用,提高了复杂产品的加工效率和精度。     

基于多体系统理论的车铣中心空间误差模型分析

数控机床的误差建模是进行机床运动设计、精度分析和误差补偿的关键技术,也是保证机床加工精度的重要环节.本文利用多体系统理论来构建超精密数控机床的几何误差模型,该模型简便、明确,不受机床结构和运动复杂程度的限制,为计算机床误差、实现误差补偿和修正控制指令提供了理论依据.在机床实际应用中,可以利用由精密机床误差建模所推导出的几何位置误差来修正理想加工指令,控制机床的实际运动,从而实现几何误差补偿,提高机床加工精度.     

基于普通车床的汽车铝合金零件多轴钻孔专机改造设计

简单介绍成组技术的基本原理,分析了应用成组技术将普通车床改造成汽车铝合金零件多轴钻孔专机的流程。对汽车泵体零件的多轴钻孔专机进行了总体结构设计,通过工艺试验确定了合理的切削用量,重点对分轴箱模块、夹具模块和控制系统模块进行了模块化设计。制造出的专机在汽车泵体等汽车零件的加工中取得了良好的效果,为企业节约了成本和时间,同时也为用闲置车床改造成加工汽车零件的高效自动化多轴钻孔专机提供了借鉴。     

NURBS-based fast geometric error compensation for CNC machine tools

In this paper, a novel method for the compensation of geometric errors of CNC machine tools is presented. The key idea is to use the basis functions of the setting NURBS path to approximate its error compensation function and to generate a new compensated NURBS path. In this way, both the setting and the compensated NURBS path have the same NURBS form. More importantly, the control points of the error compensation function can be obtained by simply calculating the positioning deviations of the control points of the setting NURBS path using the error model. A high compensation accuracy can be achieved through the systematic insertion of new knots, which creates new control points and raises the flexibility of NURBS in representing the error compensation function. The real-time interpolation of the compensated NURBS path completes the error compensation automatically. Simulations and experiments have shown that the new method delivers the same positioning accuracy as a model-based real-time geometric error compensation method does, but without additional real-time CPU loading. The proposed method can also be implemented in the post-processor of a CAM system for off-line compensation.     

A Method for Steel Ladle Lining Optimization Applying Thermomechanical Modeling and Taguchi Approaches

Successful lining concept design can avoid the premature wear of refractory linings, allow for more economically efficient configuration of refractories, and improve the efficiency of high-temperature processes and save energy. The present paper introduces the Taguchi method combining finite element (FE) modeling for the lining optimization of a steel ladle from thermal and thermomechanical viewpoints. An orthogonal array was applied to design lining configurations for FE simulations. Analysis of variance and signal-to-noise ratio were used to quantitatively assess the impact of factors on thermal and thermomechanical responses. As a result, two optimal lining concepts using commercially available materials were proposed, which showed a substantial decrease in heat loss through the steel shell and thermomechanical load at the hot face of the working lining. The combined application of FE thermomechanical modeling and Taguchi approaches facilitates the selection of proper commercial materials and thicknesses of linings for the given process conditions.     

Enhanced virtual machining for sculptured surfaces by integrating machine tool error models into NC machining simulation

Sculptured surface machining is a time-consuming and costly process. It requires simultaneously controlled motion of the machine axes. However, positioning inaccuracies or errors exist in machine tools. The combination of error motions of the machine axes will result in a complicated pattern of part geometry errors. In order to quantitatively predict these part geometry errors, a new application framework ‘enhanced virtual machining’ is developed. It integrates machine tool error models into NC machining simulation. The ideal cutter path in the NC program for surface machining is discretized into sub-paths. For each interpolated cutter location, the machine geometric errors are predicted from the machine tool error model. Both the solid modeling approach and the surface modeling approach are used to translate machine geometric errors into part geometry errors for sculptured surface machining. The solid modeling approach obtains the final part geometry by subtracting the tool swept volume from the stock geometric model. The surface modeling approach approximates the actual cutter contact points by calculating the cutting tool motion and geometry. The simulation results show that the machine tool error model can be effectively integrated into sculptured surface machining to predict part geometry errors before the real cutting begins.     

飞机多执行器液压系统的分析和改进设计

本文从国内外发展和应用的现状出发，针对飞机多招待器液压系统存在干扰的问题，介绍了一种优化设计方案，使飞机操纵系统有优先使用权。该方案对于所有飞机液压系统都适用，具有广泛的应用前景。     

Modeling system error in batch machining based on genetic algorithms

During batch machining under a static cutting-tool setting, machining errors of workpieces always have an ascending trend with the order of workpieces. Here the error trend is called the system error in batch machining. To compensate for the machining system error and enhance machining accuracy, modeling the system error is the key process. This paper first analyses several popular modeling methods and identifies their deficiencies when used in system error modeling. Subsequently, a new method of system error modeling based on Genetic Algorithms (GA) is introduced. Finally, a real-life application example using the new method and a comparison with other modeling methods are given, which indicate that the proposed GA based method is good. The paper also points out the essence that the proposed modeling method is a polynomial regression one in a broad sense, and is useful in the modeling of non-uniform error series.     

虚拟现实系统综合式框架模型的建模与系统设计

虚拟现实系统的模型结构和建模方法是虚拟现实研究领域中的一个重要内容,在对现有虚拟现实系统设计领域的成果进行分析和比较的基础上,建立了包含多构件的综合式框架模型结构,并提出了采用该模型结构进行虚拟现实系统设计的建模方法.综合式框架模型的建模方法采用分层设计方式,在每个设计阶段进一步详细划分了实现模型的步骤及所用到的技术.文中结合汽车动力总成虚拟装配系统实例介绍了采用综合式框架模型及建模方法指导并实现虚拟现实系统设计.     

An application of real-time error compensation on a turning center

A real-time error compensation (RTEC) system is developed to correct thermally-induced and geometric errors on a four-axis dual-spindle turning center. These errors vary with different cutting tool positions as well as different thermal conditions, thus real-time correction is required. Two problems in the current RTEC approach are addressed: (1) the difficulty in the actual measurement of error components according to the defined coordinates and (2) the selection of a small set of appropriate temperature variables from numerous candidate thermal sensors on a machine structure. A flexible error measurement method and an optimal temperature variable selection process are proposed to overcome these difficulties. After machine errors were characterized and modeled, the effectiveness of the developed RTEC system was evaluated by using laser inspection and an actual cutting test. The maximal diagonal displacement error is corrected from 75.0 to 7.5 μm. In the cutting test, the part diameter error of a car steering joint is reduced from 60 to 10 μm.     

多运动轴协同控制的研究

首先介绍了影响多轴控制精度的若干因素。在此基础上,着重分析了几种典型的由于轴问的相互影响和机床本身因素所导致的轮廓误差。通过分析,提出了在高速高精的加工过程中,如何通过协同控制提高系统轮廓精度。     

Prediction and identification of rotary axes error of non-orthogonal five-axis machine tool

This paper proposes an efficient and automated scheme to predict and identify the position and motion errors of rotary axes on a non-orthogonal five-axis machining centre using the double ball bar (DBB) system. Based on the Denavit-Hartenberg theory, a motion deviations model for the tilting rotary axis B and rotary C of a non-orthogonal five-axis NC machine tool is established, which considers tilting rotary axis B and rotary C static deviations and dynamic deviations that total 24. After analysing the mathematical expression of the motion deviations model, the QC20 double ball bar (DBB) from the Renishaw Company is used to measure and identify the motion errors of rotary axes B and C, and a measurement scheme is designed. With the measured results, the 24 geometric deviations of rotary axes B and C can be identified intuitively and efficiently. This method provides a reference for the error identification of the non-orthogonal five-axis NC machine tool.     

刀架几何误差模型的建立及应用

系统地分析了普通车床刀架的结构特点,以多体系统理论和齐次坐标变换原理为基础,以CA6140车床为例,采用Matlab编程建立了该车床刀架系统的几何误差模型。应用人机交互的模式可以对刀架系统进行快速的几何误差建模与分析。其误差敏感性和规律分析结果可以为刀架系统的公差设计、刚度分配提供重要依据。